About Optics & Photonics TopicsOSA Publishing developed the Optics and Photonics Topics to help organize its diverse content more accurately by topic area. This topic browser contains over 2400 terms and is organized in a three-level hierarchy. Read more.

Topics can be refined further in the search results. The Topic facet will reveal the high-level topics associated with the articles returned in the search results.

Abstract

We demonstrate a solution to make resonant-waveguide-grating sensing both robust and simpler to optically assess, in the spirit of biochips. Instead of varying wavelength or angle to track the resonant condition, the grating itself has a step-wise variation with typically few tens of neighboring “micropads.” An image capture with incoherent monochromatic light delivers spatial intensity sequences from these micropads. Sensitivity and robustness are discussed using correlation techniques on a realistic model (Fano shapes with noise and local distortion contributions). We confirm through fluid refractive index sensing experiments an improvement over the step-wise maximum position tracking by more than 2 orders of magnitude, demonstrating sensitivity down to 2 × 10−5 RIU, giving high potential development for bioarray imaging.

Figures (8)

(a) Generic resonance response in continuous and discretised form (b) Same as in (a) but in image intensity level (c, d) Shift of the resonance response resulting from a change of refractive index at the chip surface respectively for large Δn and small Δn (e) Fano shape for q parameters q = 1, q = 1.5, q = 2.5, q = 3.5 and q = 6, tending to a more symmetrical shape as q increases. Data from our structure resemble q~3.5.

(a) Gray level map of resonances as a function of micropad index and refractive index. Resonance position determined by fitting together with the correlation-based determination are reported on the map, with poorer accuracy for the two fit options. The three values indicated near n = 1.400 by yellow dashed lines indicate the reflectivities plotted in (b) for Gaussian fit and (c) for Lorentzian fit. They give poor tracking of the Fano resonance resulting in low accuracy in resonance position determination (see non-linearity of the peak position bars).

Correlation and Fano signals (a) Fano resonance simulated image for reference medium and (b) Sensed medium; (c) Signal obtained by averaging over the lines for reference and sensed medium (d) Plot of correlation (solid line) and of powers of the biased version of correlation C′10 (dots). Calculated centroid position does not have the correct position for the centre; (e) retrieval of resonance position in pixel shift units. The centroid of C′10 has the right slope, whereas the centroid of C has its slope flattened; (f) Relative slope accuracy ΔS/S as a function of k and q in the case of C′k (solid contours) or in the worse case of Ck (dashed contours).

Simulated reference and sensing tracks in the presence of a large noise (signal-to-noise ratio ~1 at the pixel scale) (a) for reference track and (b) for sensed track; (c) Signal obtained by averaging over the lines for reference and sensed media; (d) Plot of correlation C (solid line) and of power of the biased version of correlation C′10 (dots) ; (e) retrieval of resonance position in pixel shift units. The rms fluctuation in pixel units is 0.79 in the former case, with also a still inadequate slope, and 0.44 in the second case.

(a), (b) Distortion simulation. Each micropad is assumed to suffer from an internal distortion of its resonance position corresponding to 3.5 micropad unit, with the smooth normalized pattern shown in (e). (c) Projection on y averaged over the lines (d) Correlation functions C and C′10. (e) Distortion map. (f) Retrieval performances through the centroids of functions C and C′10. The rms noise remains below the tenth of pixel limit.

(a) Scheme of the experimental setup composed of a source monochromatically filtered and polarized, illuminating a chip with 2 tracks, one serving as reference and the other for sensing, and a camera to image the chip (b) Measured images of one of the reference picture and media with index from n = 1.333 to n = 1.474 (c) Reflectivity profiles for each of the reference (blue) and of each of the media (n = 1.333 to n = 1.474) (d) Reported peak position determined by Gaussian fit (green), Lorentzian fit (cyan) and correlation analysis (red). The measured resonance position is plotted in abscise axis and the known refractive index of the solution in ordinate axis.

(a) Images of tracks for micropads from 24 to 36, both for reference and sensed solution for indices from 1.333 to 1.337 by step Δn = 10−3 (b) Experimental profiles using central pixels of micropads and plotted as line profile for visual convenience (c) Normalized correlation C (greenish broad bell-shaped) and C′10 (brownish narrower) curves, as well as centroid center position (reddish vertical lines).(d) Shift of the peak determined by Gaussian and Lorentzian fits as well as correlation analysis. Different fits and the correlation method give aligned points whose fitted slope can be used as index transduction calibration. The inset gives the error with respect to the fitted slope trend, plotted in abscissa vs. the analyte refractive index (ordinate), for all three analyses. Errors are on the order of a few 10−5, thus a few percent of the 10−3 index step.